Field of the Disclosure:
The present disclosure generally relates to the field of retrieval of objects submerged in a body of water including objects positioned at the bottom of a body of water, as is important in the field of Anchor Handling, and more particularly to the field of pre-laid anchor systems, and yet more particularly to the field of pre-laid anchor systems for use in exploration drilling of fossil fuels, and yet even more particularly to the field of pre-laid anchor systems for use in exploration drilling of fossil fuels where fiber ropes are used in long spans in substitution of long spans of chain and steel wire.
Background Information
Anchor handling, especially in winter, can be time-consuming and therefore costly. The operations often have to wait for a suitable weather window. The probability of finding a weather window to moor a rig to a pre-laid anchor system is about sixty percent greater than the probability to find a weather window to anchor the rig as it arrives. Delays and operational downtime associated with setting a rig's anchor system as a rig arrives in place can run into millions of dollars per rig set. In addition to the economic costs, anchor handling is one of the most dangerous operations during exploration drilling. The worse the weather conditions, the greater the costs and the greater the danger.
Thus, it has become important that the anchor system for a rig is pre-laid during safe weather conditions so that when a rig is ready to be moved into place the anchor system for the rig is already laid in place and ready for connection to the rig.
Faced with increasingly deeper installations, rig mooring lines in most new drilling sites are mainly formed with fiber rope for the reduced weight afforded by the fiber rope in comparison to chain or steel wire. The lesser the weight of the mooring lines the resultantly less floatation needed to suspend the mooring lines, permitting more economical rig constructions. Problematic to pre-laying anchor systems that mainly use fiber rope is that fiber ropes are easily damaged from contact with submerged obstacles, including other mooring lines and existing and abandoned infrastructure.
In attempt to solve these problems, it has become state of the art to pre-lay all portions of a rig's anchoring system with the exception that the fiber rope portion of the anchoring system is stored in a container that is stored at the seabed, especially in a large fabric bag or other container that is stored on the ocean or sea bottom. The fiber rope portion of the anchoring system is then retrieved just prior to connection to the rig.
Published PCT application having publication number WO2011102730, titled “IMPROVED DEVICE AND METHOD FOR FORMING AN ANCHOR SPREAD” teaches the present state of the art and trend in the industry for pre-laying rig anchor systems using fiber rope mooring lines. As taught in this publication, anchors are put out in advance; each of the anchors is connected to a fiber rope that is wrapped in a special way and stored in a “bag” or other container on the seabed. In addition, a signal buoy, such as a signal buoy know as a “Spin Buoy” can be used to lighten the connection with the fiber rope to the signal buoy. When at a later date the rig is in place at the location, a vessel can “call on” the signal buoys using coded sound waves, such as may be transmitted from an ROV. The signal buoys then rise to the surface while trailing behind each signal buoy a signal buoy line that ultimately connects to the much stronger fiber rope mooring line. The vessels can then collect the signal buoys, haul in the signal boy lines and thus retrieve ends of the fiber rope mooring lines, and at the retrieved end connect the fiber rope mooring lines with the rig's chains.
Recent state of the art attempts to store a fiber rope near the seabed for later retrieval include storing the fiber rope by winding it both upon as well as external a buoyant structure that is essentially a buoyant spool, spindle or cylinder, with one end of the fiber rope attached to the buoyant spool, spindle or cylinder (hereinafter also referred to as a “buoyant spool”) and with the other end of the fiber rope, that is the last part of the fiber rope to be wound upon the buoyant spool, being attached to a chain that serves as an anchor chain that attaches to an anchor, the anchor thereby holding the end of the fiber rope to the sea bed. The buoyant spool is also fixed to the seabed by means of also being connected to, for example, the anchor chain, by means of a trigger that can be remotely activated by, for example, a coded acoustic signal. Upon activation, the trigger releases, and the buoyant spool ascends to the surface. The natural reaction of the upward ascent force of the buoyant spool, countered by that downward restraining force of that end of the fiber rope that is anchored to the seabed is to cause the buoyant spool to rotate about its long axis as it ascends, thereby paying out the fiber rope wound upon the cylinder. This rotation, induced during paying out of the signal buoy line for reasons taught supra, is known as “spinning”. The present state of the art for such a spool, spindle or cylinder is known as the “Spin Buoy” and is promoted by Viking Moorings. In the present state of the art and current trend of the industry the rope coiled about a portion of the generally cylindrical signal buoy and/or fiber rope storage buoy (i.e. the buoyant generally cylindrically shaped spool with flanged ends, and e.g. a Spin Buoy), is especially a braided rope having a cross section that has an aspect ratio that preferably is one to one, and is either one to one, or less than one and one half to one. It is considered important that the fiber ropes cross section be as close to one to one as possible in order to preclude tangles and backlashes during payout while the buoy is spinning.
A result of the spinning is that Magnus Effect forces are caused by the rotation of the buoyant cylindrically shaped spool during its ascent through the water, and generate forces largely normalized relative to a straight line directed from the anchor point of the buoyant spool directly upwards against gravity toward the surface of the body of water.
In addition to the spinning, the downward vector acting upon the buoyant spool that is resulting from the downward pull of the fiber rope stored upon and being unwound from the buoyant spool is constantly varying it's point of origination on the spool, spindle or cylinder during its ascent to the surface due to the fact that as the fiber rope unwinds, it is continually travelling across the long dimension of that portion of the buoyant spool upon which it is spooled, thereby changing what point upon the axis of the buoyant spool originates the downward force vector resulting of the fact that the fiber rope is anchored to the bottom or to a submerged object at one of its ends and is connected to the buoy at another of its ends. As a result, the buoyant spool is constantly varying its orientation relative to gravity during it ascent through the water and to the surface.
Problematically, this constant variation of the orientation relative to gravity, in combination with the generated Magnus Effect forces, causes the buoyant spool to be subject to a great variety of fluxing lateral forces during it's ascent to the surface. The result of the Magnus Effect's horizontal forces is to cause the rotating buoyant cylindrically shaped spool to deviate from what would be a straight line ascent to the surface, and when combined with the fact that the buoyant spool is constantly varying its own orientation relative to gravity, the result of these combined phenomenon is that this type of signal buoy is constantly moving laterally in both the Y and Z directions during its ascent. Consequently, rather than breaching the surface at a certain location, these types of signal buoys are breaching at unexpected locations. Because much kinetic energy is present in the breaching signal buoy, which forces can easily kill a person, the unpredictable location of the buoy's breach presents a danger to crews' safety and lives.
However, “Spin Buoys” continue to be used in the industry, both as “Signal Buoys” and also as storage buoys for retrieval of mooring lines, despite the danger to crews, because winding a fiber rope on the external and generally cylindrical portion of a buoyant structure that is later called upon and remotely triggered or released so it can ascend to the surface from at or near a seabed is considered by the industry to be the most reliable way to store and subsequently unwind and/or pay out without tangles and backlashes a fiber rope upon a structure that is ultimately called upon to ascend to the surface from a previous location at or near a seabed.
Thus, it can readily be appreciated that a need continues to exist in the industry for a signal buoy or fiber rope storage buoy that breaches the surface in a reduced region of the surface so as to reduce the total potential area of its breach at the water's surface, thus increasing predictability of the signal buoys' breach location, thereby reducing danger to crews.
Thus, it also can readily be appreciated that a long felt need continues to exist in the industry for a solution to the problem of storing the fiber rope portion of a pre-laid anchoring system where the fiber rope portion of the pre-laid anchoring system can be safely retrieved.
None of the known art has proposed a solution to the above stated long felt needs of industry that is same as the solution taught in the present disclosure.